1. Department of Organic and Biomolecular Chemistry, University of Göttingen, Tammannstraße 2, D-37077 Göttingen, Germany
2. Department of Chemistry, University of Venda, P/Bag X5050, Thohoyandou 0950, South Africa
3. Department of Plant Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa